1. Field of the Invention
The present invention generally relates to an adjustable resistor embedded in a multi-layered substrate and method for forming the same and, more particularly, to an adjustable resistor and a method for forming the adjustable resistor using a mechanical drilling process to selectively break the connecting lines to form a number of combinations of opened connecting lines such that the resistance value of the adjustable resistor can be precisely adjusted.
2. Description of the Prior Art
It is well known that resistors have been widely used in circuit applications such as restrictors, regulators and terminator controllers, etc. however, most small-size resistors are implemented using surface mounted technique (SMT). Even though resistors are developed to be smaller and more compact, they still have to be installed on the surface of multi-layered substrate, which increases the area and height of the physical layer. In order to embed the resistors in the multi-layered substrate, there have been many reports on the circuit applications of coated resistors. However, the coated resistors require a precise laser mechanism or a sand-blasting grinder to perform fine-tuning because of the resistance inaccuracy due to improper recipe and screen printing. Moreover, such a method is only effective for the fine-tuning of surface coated resistors.
For commercially available thick-film resistors (TFR's), the printing ink is unstable when it is coated on the substrate. Therefore, a fixed resistance value does not exist. The variance between the resistance value and the estimated value is about ±20%. In order to precisely obtain the designed resistance value, an additional trimming process after the formation of the thick-film resistors is required. The additional trimming process includes sand-blasting trimming and laser trimming.
Sand-blasting trimming uses a nozzle to discharge an abrasive with a high-pressure gas. The thick-film resistor layer is grinded with the abrasive so as to trim the shape of thick-film resistor layer. Sand-blasting trimming is advantageous for its non-thermal process. However, sand-blasting trimming leads to exposure of the resistor edge and powder contamination and fails to achieve high-precision trimming. Laser trimming uses the heat generated from laser so as to oxidize the material of the resistor layer and form a gap of 2 to 2.5 mil. Laser trimming is advantageous for its high elimination rate, computer-controllable automation, precise and fast treatment. Moreover, laser trimming can be performed on a high-integrity circuit board. However, it is also problematic for its cracks due to thermal shock.
In U.S. Pat. No. 4,443,782, entitled “Method for Regulating the Value of a Thick Film Resistor and a Corresponding Resistor”, a plurality of slots in the resistor material region are used to adjust the resistance value of the surface coated resistors. Even though U.S. Pat. No. 4,443,782 does not disclose how the slots are formed, laser trimming is mostly used to adjust the resistance value.
The conventional technology related to surface coated resistors cannot be applied to embedded resistors. Therefore, there exists a need in providing an adjustable resistor embedded in a multi-layered substrate and method for forming the same. The adjustable resistor comprises: a planar resistor, having a plurality of terminals; and a plurality of connecting lines connected to the planar resistor, each of the connecting lines being drawn from each of the terminals of the planar resistor so as to form a resistor network, wherein the connecting lines are selectively broken by a process for drilling the substrate to form a number of combinations of opened connecting lines such that the resistance value of the adjustable resistor is varied and thus the resistance value of the adjustable resistor can be precisely adjusted.